Efficient and quiet operation are important requirements for vacuum cleaners and similar air moving devices. To move the working fluid, i.e., air, within the vacuum cleaner an impeller is generally rotated within a housing by an electric motor. The impeller draws air at a central location and expels air about its circumference. The housing encloses the impeller and generally includes an air inlet passage located adjacent a central location on an inlet side of the impeller and a discharge passage extending about the circumference of the impeller.
It is known that some resistance to the free discharge of air from the impeller, or back pressure, increases its operating efficiency. Vanes are frequently provided disposed about the circumference of the impeller and within the discharge passage to provide back pressure. The vanes may have an involute configuration and are generally arranged to at least partially restrict the air flow. One disadvantage arising from the use of vanes in this manner is that they tend to generate noise. Air is a compressible fluid. As an impeller blade passes adjacent a leading edge of a vane the air is compressed and rapidly decompressed creating pressure pulses, i.e., noise. This noise is objectionable particularly in a vacuum cleaning device that may be used in a home or workshop.
Past proposals for eliminating or reducing noise include placing a dome structure over the housing into which the air is discharged. The large plenum created by the dome structure and the indirect pathway between the housing discharge passage and an air exit in the dome structure cooperate to reduce noise. Another proposal uses complex involute passages formed in the housing adjacent the circumference of the impeller. The involute passages are intended to provide smooth air flow and an increasing volume into which the discharge air decelerates to static pressure. These and other proposals, while offering varying degrees of success in maintaining impeller operating efficiency and reducing noise, have not been entirely satisfactory. Furthermore, a number of these proposals have added to the overall size of the impeller housing, and the complex involute passage proposal particularly adds to the manufacturing cost and complexity of the vacuum cleaner product.